Methods and apparatus for distilling a liquid mixture, generally make use of tower-like columns. It is, however, also known to use stripper columns in the form of cylindrical towers in which the liquid mixture to be separated is introduced through the top portion and flows downwardly while vapor generated at the bottom ascends in countercurrent flow. Stripper columns are in general designed as plate columns in which horizontal plates or trays are used to provide the exchange of constituents between the liquid and the vapor fraction, or as packed columns which use packings to provide the exchange of constituents (material exchange) by bringing the liquid and the vapor into intimate contact with each other.
These known columns have the drawback that they must be of considerable height since the separation of liquid and vapor depends largely on the action of gravity.
Two conditions must be taken into account in dealing with a plate or tray column. First, the flow of the liquid over the horizontal plates is obtained by the liquid pressure build-up which is provided through the laterally arranged weir of the column and the magnitude of the pressure build-up is proportional to the height of the liquid column and the effective acceleration due to gravity.
Second, the liquid droplets which are upwardly expelled from the liquid boiling over the plate should not be entrained by the ascending vapor to the next upper plate. It is obvious that the maximum vapor speed to realize this is proportional to the vertically downwardly directed acceleration and increases correspondingly with the acceleration. Thus, both conditions necessitate a substantial height for such stripper columns.
It should now also be apparent that stripper columns of given output can be smaller if the effective acceleration is greater.
In practice, moreover, it has been found that conventional stripper columns do not operate satisfactorily under conditions of excessive formation of foam. Likewise, there may be occasions e.g. upon high pressure that the density of the vapor is almost the same as the density of liquid so that a separation of the two phases is difficult in stripper columns whose effectiveness is based on gravity.